1. Field of the Invention
This invention relates generally to novel radiation-sensitive negative-working surface-sensitive emulsions and materials exhibiting reduced pressure sensitivity. More particularly, this invention relates to photographic emulsions and materials incorporating silver halide grains of a structure and composition which imparts to them a reduced tendency towards density formation when they have been subjected not to radiation exposure but to pressure.
2. Description and Limitations of the Prior Art
Black pressure sensitivity (or pressure sensitization) is the formation of density on a radiation-sensitive emulsion or material (after the emulsion or material has been processed) in areas where the emulsion or material was subjected to mechanical pressure.
Black pressure sensitivity manifests itself in the form of black smudges or black tracks or in more irregular black patterns which are usually encountered (and quite noticeable) in non-exposed areas of the emulsion or material.
This defect is most common in photographic materials which are handled in sheet form (such as photographic film or phototypesetting paper materials) or which are scratched, kinked or squeezed between guide rollers and other equipment while in a pre-processed state (and therefore vulnerable to density formation) such as during their manufacture and handling. This defect is particularly pronounced in homogeneous silver bromide emulsions doped with a Group VIII metal.
White pressure sensitivity is density inhibition in response to pre-exposure application of pressure, and manifests itself by the presence of white marks where the photographic material has been subjected to pressure and then exposed.
3. Description of the Prior Art
U.S. Pat. No. 4,710,455 (Dec. 1, 1987) discloses monodisperse cubic silver halide emulsions of the so-called core/shell type. However, contrary to the present invention, the '455 emulsions are made by initially preparing a first silver halide crystal followed by precipitation of a second silver halide having a lower solubility than the first halide so as to dissolve the first halide crystals and result in monodisperse second silver halide crystals having depressions and hollows. No pressure sensitivity reduction is mentioned. The resulting crystals are said to have improved ability to adsorb spectral sensitizing dyes (including red spectral sensitizing dyes).
U.S. Pat. No. 4,963,467 (Oct. 16, 1990) is directed to core/shell emulsions wherein the core has a minimum 10% silver iodide content. As explained further below, the pressure sensitivity of the present invention is adversely affected by the presence in the core and/or the shell of substantial amounts of iodide (e.g. higher than about 1%).
U.S. Pat. No. 4,952,485 (Aug. 28, 1990) discloses silver halide color negative photographic materials which are said to have improved pressure resistance. The emulsions said to be incorporated in these materials all contain core-shell crystals with an iodobromide core containing more than 5% iodide and a silver iodobromide shell containing less iodide than the core.
U.S. Pat. No. 4,965,176 (Oct. 23, 1990) discloses internal latent-image direct-positive silver halide core/shell emulsions containing "at least silver chloride" which are developed in a solution containing a compound of the formula R.sub.1 R.sub.2 (N)OH. No Group VIII metal doping is mentioned. The core is gold-sulfur (chemically) sensitized and the shell is sensitized with sulfur only. The balance between internal and external sensitivity is said to be critical.
U.S. Pat. No. 4,977,075 (Dec. 11, 1990) discloses twinned tabular (aspect ratio .gtoreq.2:1) emulsions of allegedly improved pressure properties. The matrix of the crystals contains at least 50 mole % bromide. The top layer (mostly deposited on basal planes) is a silver halide layer containing at least 50 mole % silver chloride. The matrix can contain up to 30 mole % silver iodide.
U.S. Pat. No. 4,070,190 (Jan. 24, 1978) discloses silver halide emulsions having a core/shell structure made by forming silver chloride grains, converting them to silver bromide grains (i.e. removing chloride ions from the crystallization process) and then precipitating on the silver bromide grains a silver chloride shell. By contrast, no conversion takes place in the process of the present invention, and this can be substantiated experimentally.